256 



SCIENCE OF GARDENING. 



PART II. 



point of observation, the column of air over a barometer placed at that level is the longest 

 to be obtained. 



The variations of the barometer between the tropics are very trifling, and it does not descend more than 

 half as much in that part of the globe for every two hundred feet of elevation as it does beyond the tropics. 

 The range of the barometer increases gradually as the latitude advances towards the poles, till in the end . 

 it amounts to two or three inches. The following Table will explain this gradual increase : 



The range of the barometer is considerably less in 

 North America than in the corresponding latitudes of 

 Europe, particularly in Virginia, where it never ex- 

 ceeds I'l. The range is more considerable at the level 

 of the sea than on mountains ; and in the same degree 

 of latitude it is in the inverse ratio of the height of the 

 place above the level of the sea. Cotte composed a 

 table, which has been published in the Journal de Phy- 

 sique, from which it appears extremely probable, that the 

 barometer has an invariable tendency to rise between 

 the morning and the evening, and that this impulse 

 is most considerable from two in the afternoon till nine 

 at night, when the greatest elevation is accomplished ; 

 but the elevation at nine differs from that at two by 

 four twelfths, while that of two varies from the elevation of the morning only by one twelfth, and that in 

 particular climates the greatest elevation is at two o'clock. The observations of Cotte confirm those of 

 Luke Howard ; and from them it is concluded, that the barometer is influenced by some depressing 

 cause at new and full moon, and that some other makes it rise at the quarters. This coincidence is most 

 considerable in fair and calm weather ; the depression in the interval between the quarters and conjunc- 

 tions amounts to one tenth of an inch, and the rise from the conjunctions to the quarters is to the same 

 amount. The range of this instrument is found to be greater in winter than in summer ; for instance, the 

 mean at York, during the months from October to March inclusive, in the year 1774, was T42, and in the 

 six summer months 1-016. 



The more serene and settled the weather, the higher the barometer ranges ; calm weather, with a tendency 

 to rain, depresses it ; high winds have a similar effect on it ; and the greatest elevation occurs with easterly 

 and northerly winds j but the south produces a directly contrary effect. 



1230. The variations in the temperature of the air in any particular place, exclusive of 

 the differences of seasons and climates, are very considerable. These changes cannot be 

 produced by heat derived from the sun, as its rays concentrated have no kind of effect on 

 air ; those, however, heat the surface of our globe, which is communicated to the immediate 

 atmosphere ; it is through this fact that the temperature is highest where the place is so 

 situated as to receive with most effect the rays of the sun, and that it varies in each region 

 with the season ; it is also the cause why it decreases in proportion to the height of the 

 air above the surface of the earth. The most perpendicular rays falling on the globe at 

 the equator, there the heat of it is the greatest, and that heat decreases gradually to the 

 poles, of course the temperature of the air is in exact unison ; from this, it appears, that 

 the air acquires the greatest degree of warmth over the equator, where it becomes insensi- 

 bly cooler till we arrive at the poles ; in the same manner, the air immediately above 

 the equator cools gradually. Though the temperature sinks as it approaches the pole, 

 and is highest at the equator, yet as it varies continually with the seasons, it is impossible 

 to form an accurate idea of the progression without forming a mean temperature for a 

 year, from that of the temperature of every degree of latitude for every day of the year, 

 which may be accomplished by adding together the whole of the observations and dividing 

 by their number, when the quotient will be the mean temperature for the year. The 

 ' diminution," says Dr. Thomson, " from the pole to the equator takes place in arith- 

 metical progression ; or to speak more properly, the annual temperature of all the lati- 

 tudes, are arithmetical means between the mean annual temperature of the equator and 

 the pole. And as far as heat depends in the action of solar rays, that of each month is as 

 the mean altitude of the sun, or rather as the sine of the sun's altitude." 



1231. Inconsiderable seas, in temperate and cold climates, are colder in winter and 

 warmer in summer than the main ocean, as they are necessarily under the influence of 

 natural operations from the land. Thus the Gulf of Bothnia, is generally frozen in 

 winter, but the water is sometimes heated in the summer to 70, a state, the opposite part 

 of the Atlantic never acquires ; the German Sea is five degrees warmer in summer than 

 the Atlantic, and more than three colder in winter ; the Mediterranean is almost through- 

 out warmer both in winter and summer, which therefore causes the Atlantic to flow into 

 it ; and the Black Sea being colder than the Mediterranean, flows into the latter. 



The eastern parts of North America, as appears from meteorological tables, have a much colder air than 

 the opposite European coast, and fall short of the standaid by about ten or twelve degrees. There are 

 several causes which produce this considerable difference. The greatest elevation in North America is 

 between the 40th and 50th degree of north latitude, and the 100th and 110th of longitude west from Lon- 

 don ; and there the most considerable rivers have their origin. The height alone is sufficient to make this 

 tract colder than it would otherwise be ; but there are other causes, and those are most extensive forests, 

 and large swamps and morasses, each of which exclude heat from the earth, and consequently prevent it 

 from ameliorating the rigor of winter. Many extensive lakes lie to the east, and Hudson's Bay more to 

 the north ; a chain of mountains extends on the south of the latter, and those equally prevent the accu- 

 mulation of heat ; besides, this bay is bounded on the east by the mountainous country of Labrador, and 

 has many islands ; from all which circumstances arise the lowness of the temperature, and the piercing 

 cold of the north-west winds. The annual decrease of the forests for the purpose of clearing the ground 



